On Monday, the U.S. Navy will officially announce the ships for its demonstration of the “Great Green Fleet” — an entire aircraft carrier strike group powered by biofuels and other eco-friendly energy sources. If a powerful congressional panel has its way, it could be the last time the Navy ever uses biofuels to run its ships and jets.

In its report on next year’s Pentagon budget, the House Armed Services Committee banned the Defense Department from making or buying an alternative fuel that costs more than a “traditional fossil fuel.” It’s a standard that may be almost impossible to meet, energy experts believe; there’s almost no way the tiny, experimental biofuel industry can hope to compete on price with the massive, century-old fossil fuels business.

Committee Republicans, like Rep. Randy Forbes, insist this isn’t an attempt to kill off military biofuels before they have a chance to start. “Now, look, I love green energy,” he said in February. “It’s a matter of priorities.”

But if the measure becomes law, it would make it all-but-inconceivable for the Pentagon to buy the renewable fuels. It would likely scuttle one of the top priorities of Navy Secretary Ray Mabus. And it might very well suffocate the gasping biofuel industry, which was looking to the Pentagon to help it survive.

“We’d be years behind if it wasn’t for the military,” said Tom Todaro, a leading biofuel entrepreneur whose companies have supplied the military with tens of thousands of gallons of fuel made from mustard seeds.

When Mabus took over as Navy Secretary, he declared that the service would get half of its energy from sources other than oil by 2020. The two-day Great Green Fleet demo, scheduled for the end of June in Hawaii, is supposed to be the biggest step yet towards that beyond-ambitious goal.

The destroyers USS Chafee and Chung Hoon will plow through the Pacific and F/A-18 jets will scream off of the USS Nimitz‘s flight deck, all thanks to a 50/50 blend of alternative and traditional fuel. It’ll not only show the world that the Pentagon is serious about biofuels — a full-scale Green Fleet deployment is scheduled for 2016. It’ll also serve as a signal to skittish investors that biofuel companies have a willing customer in the U.S. Navy.

There were political costs, too. Committee Republicans — unhappy about shrinking defense budgets and skeptical about the White House’s green initiatives — used the biofuel buy as a way to go after the administration.

“I understand that alternative fuels may help our guys in the field, but wouldn’t you agree that the thing they’d be more concerned about is having more ships, more planes, more prepositioned stocks,” Rep. Randy Forbes said during a February hearing with Mabus. “Shouldn’t we refocus our priorities and make those things our priorities instead of advancing a biofuels market?” Then he told Mabus: “You’re not the secretary of the energy. You’re the secretary of the Navy.”

Mabus and his allies countered that the Republicans were taking an overly-simplistic view of things. Of course relatively small batches of a new fuel are going to be expensive — just like the original, 5GB iPod cost $400 and held fewer songs than today’s $129 model, which holds 8 GB. That’s the nature of research and development. With development time and big enough purchases, the costs of biofuels will come down, they argued; already, the price has dropped in half since 2009.

“It’s a false choice to say that we should concentrate on more ships versus a different kind of fuel. If we don’t get a different kind of fuel, if we don’t have a secure domestic supply of energy at an affordable price… the ships and the planes may not be able to be used because we can’t get the fuel,” Mabus told the Senate Subcommittee on Water and Power in March.

What’s more, Mabus added, there’s a value in a more stable, domestic supply of fuel; every time the price of oil goes up by a dollar per barrel, it costs the Navy $31 million. “We simply buy too much fossil fuels from places that are either actually or potentially volatile, from places that may or may not have our best interests at heart,” he said. “We would never let these places build our ships, our aircraft, our ground vehicles, but we do give them a say on whether those ships steam, aircraft fly, or ground vehicles operate because we buy so much energy from them.”

None of those arguments managed to sway House Republicans, who last Wednesday voted to impose its ban on alt-fuels that cost more than the traditional stuff. InsideDefense.com first noted the measure.

Long before the congressmen made their decisions, biofuel industry insiders told Danger Room that their products would never be as cheap as petroleum-based ones.

“This idea that we can match [the price of] crude oil — I think it’s such a bullshit question,” Tom Todaro said back in October. “A car with airbags costs more than a car without. Society decides how valuable those airbags are. Society can decide the value of renewable fuels.”

During his testimony in March, Mabus insisted that “the Great Green Fleet doesn’t have an environmental agenda. It’s about maintaining America’s military and economic leadership across the globe in the 21st century.” Still, it’s hard to imagine him agreeing to a Great Green Fleet that polluted the planet even further.